Techniques for open forming of articles of synthetic resin encompass a number of different processes, e.g. spray-up, hand lay-up, centrifugal casting, open casting and multifilament winding, for preparing structural products, architectural decorative pieces, lamp bases, furniture parts, immitation marble sink tops, shower enclosures and tubs, boats, fluid tanks, microwave antennas, sewage pipes, etc.
Prior art foam systems are not generally used in such structural products, a major field, but are used mostly in the limited cases where a low density core or backer is desired for stiffening and similar purposes. In particular, polyester resin, which has many advantages in rigid structural parts, has not been effectively utilized in expanded state.
In a typical product that does have a foam layer, used e.g. as a substitute for balsa or plywood, the foam layer, typically of quite low density, is laminated to a preformed layer of fibreglass-reinforced, nonexpanded synthetic resin. Another nonexpanded layer may be provided on the other side of the low density layer. Each layer is applied to the mold separately. A finish gel coat, where used, typically is provided as the initial coating of the mold and is backed by a nonexpanded, separately applied resin layer prior to application of the low-density foam layer. Typically, foam is spaced well away from the finished surface and generally does not contain reinforcement fibres. If reinforcing fibres are used, the fibres are typically encapsulated in groups without being thoroughly penetrated and covered by the resin, and hence their reinforcement effect is relatively inefficient.
In certain foam systems that are used for cores, such as certain known urethane-foaming reactions, the ingredients must be combined stoichiometrically within a critical ratio of 1% by a precise mixing device. However, because such known reactions occur so quickly, it is necessary that the mixture be ejected from the device immediately. These considerations have caused known systems to employ accurate and expensive pumps, e.g. gear pumps, to deliver the ingredients at a smooth, precise and constant flow rate. The quick reactions of prior systems also make it necessary to provide solvent or mechanical purge systems to periodically flush or push reacted material out of the reaction chamber. This solvent handling and the purge cylinder add additional costs to apparatus to handle these systems.
Also inherent to the quick foaming reaction of certain prior systems is the fact that gelation does not occur quickly enough to form or preserve bubbles of uniform size and shape. The gas bubbles in these systems tend to become irregular and may be at or near the surface, thereby weakening it. This prevents use as finished surfaces and in direct support of gel coats.
Ideally a resin expansion system should be inexpensive and require inexpensive equipment, should utilize components that are nontoxic and have a long shelf life (both individually and when mixed with other resin system components), and should be capable of achieving a full range of cellular densities. It is desirable that the gas-generating components be capable of functioning over a non-critical range of proportions without harming or altering the composition of the product.